Parts: 0.1uF decoupling capacitors

Most ICs need to be decoupled from their power supply, usually with a 0.1uF capacitor between each power pin and ground. Decoupling is usually used to remove noise and to smooth power fluctuations. Every project will need a few decoupling capacitors; our mini web server project has three ICs that require a total of 11. This can be an expensive part to buy in singles, so it’s crucial to stock up online. Read more about our favorite bulk through-hole and surface mount decoupling capacitors after the break.

The capacitors we selected should be sufficient for most projects. All three parts are rated for 50volts, far more than most digital circuits. We used cheap 20% tolerance parts because it’s not critical that decoupling capacitors be exactly 0.1uF. Higher or lower tolerance capacitors will also work, but there’s no advantage to using high quality decoupling capacitors. Here’s a breakdown of the 0.1uF capacitors pictured above:

C1through-hole 0.1uF capacitor, such as Mouser #594-K104M15X7RF53L2, ($4 per 100 ) – This cheap 0.1uF capacitor will fit almost any design that calls for through-hole decoupling capacitors. Leads are spaced 2.5mm apart, and fit footprints such as C-EU025-025×050 in the default Cadsoft Eaglercl library. Part number -L2 has straight legs, -K2 has an outside kink as shown in the picture.

C21206 SMD 0.1uF capacitor, such as Mouser #77-VJ12Y50V104K, ($4 per 100) – You might be tempted to try 1206 size parts in your first surface mount experiments. We urge you to skip 1206 and head right on down to 0805. 1206 parts have very little cost benefit over through-hole parts because they’re no longer an industrial favorite. 0805 is just a bit smaller, but costs half as much. Fits Eagle footprint C-EUC1206 in the default rcl library.

C30805 SMD 0.1uF capacitor, such as Mouser #80-C0805C104M5R, ($2 per 100) – An 0805 capacitor fits between two pins on a through-hole DIP chip, and aligns nicely with pins on an surface mount SOIC chip. All our new designs, surface mount and through-hole, incorporate this dirt-cheap decoupling capacitor. Fits footprint C-EUC0805 in the default Eagle rcl library.

Yep! Real handy when the IC has its power pins right next to each other. Solder one through-hole pin. Tweezer your cap close to in-place, heat that solder and tweeze the cap right inbetween the two pins, then solder that other pin. Great for breadboard prototyping.

Was an anti-SMT person for some time. Glad I reconsidered. The space-requirement for through-hole passives is ridiculous!

and typically at what complexity should you start decoupling a circuit. For example if I threw a few led’s in with a simple controller should I worry or should you start adding these caps anytime you have an IC involved? I’m new to making “correct” circuits…

Power supply regulation doesn’t really address the same problem as decoupling caps – think of decoupling caps as local energy reservoirs for local /fast/ switching currents. PS regulation is usually very slow by comparison.

The reason they need to be local is the impedance of the power supply network. At higher frequencies, the power supply network has too much impedance for a central regulation or cap bank to work. As the frequencies lessen, the area that can be decoupled by one cap effectively grows. SMT helps by decreasing the lead inductance in the caps and is most important for the small caps.

In big boards a multiple tier strategy is used – small caps right next to the IC, bigger caps spaced out over the board, and a few really big caps usually near the power entrance.

The edge rate is most important – a chip that switches really fast (more dV/dt) will cause more noise per transition, so for example 74AC logic will cause more problems than 74LS because it switches faster.

They don’t cost much, so I would err on the side of caution and put them in.

P.S. One reason I got my first real job was because on an interview question, while drawing a circuit, I remembered to put in a power supply block and decoupling :-)

If you’re just stocking up for prototyping and some of your circuits require it very good noise decoupling, you might want to stock 0306/0608/0812’s instead. They have considerably better decoupling characteristics because the rotated geometry actually considerably reduces parasitic inductance. I’d also imagine the larger surface area for soldering would also make it easier to solder and allow for some of more “creative” mounting methods.

@haku: I’ve had similar experiences, people don’t realize that it’s not as hard as it seems to solder the little guys. I also concur on the 0603/0804 replacement, it works astonishingly well. Its possible to go the other way if you’re in a really tight pinch, but I’ve only managed it once or twice.
0201s on the other hand, I just don’t understand them. With my naked eye I can barely see the distinct pads.

As for future articles, one on pull [up|down] resistors would be fitting. I think that’s a trouble spot for beginners.
In a similar vien, possibly a board layout article? Not using the software but raising awareness (heh) of crosstalk & the like. I’ve seen some hobbist pcbs encounter small problems due to bad PCB design. (OTOH, I’ve seen overly-optimized boards as well…)

Regulators keep your voltage stable, but they are rather slow. Whenever a part on your board requires more current, the voltage will drop a bit, so the regulator needs to … regulate. In some cases (switching port pins) a µC needs a “lot” of current for a short time.

Without the decoupling, this would be drawn from the whole power plane, resulting in a voltage drop which needs to be compensated by the regulator.

With decoupling (assumed the capacitor is close to the Power pins), this high current loop is a lot smaller.

@Hack-A-Day: Maybe you guys could make an article about different step-up-regulators(especially the kind which can power 3.3/5V devices from a single cell).

Perhaps a series of followups describing different types of IC’s including logic (4000 series, 7400 variants such as LS, HC, etc) and linear (opamps, comparators, regulators, other specifics). Another set of followups would be covering things like uC fundamentals, and on the other end of the spectrum, things like project design (power usage, ergonomics, cost, design considerations such as design from schematic to PCB to enclosed project).

Last but not least articles covering power switching such as bipolar transistors, MOSFETs, SCRs/TRIACs, and IGBTs.

Nice tutorials you’re making these days! I think you should address power consumption issues, so designers can be aware that a little improvement in a design can save a lot of power on a large scale. A credit card sized web server is cool, but how about “a credit card sized web server that can run a week on a lemon”? :)

Reflow toaster-oven method:
1) Manually remove the electrolytics and other parts that are likely to melt
2) Put board on drip pan – parts side down
3) Set reflow toaster oven to about 400F
4) When parts start falling off, open door and hit the board with a small screwdriver to release parts

Redneck Method:
1) Go outside
2) Put a board on drip pan – parts side down
3) Heat the back of the board with a propane torch
4) Leave area, collect parts when the smoke clears